"They were very playful," an observer tells Us. "Eva was skipping around like a little girl...She would lean into him and she held his arm the entire time."

And the sexy twosome did Disneyland right -- riding the Toy Story ride, the California Screamin' rollercoaster, a Little Mermaid adventure and the ferris wheel, snacking on churros, cotton candy and corn on the cob. "She fed him," the onlooker says.

Currently shooting The Place Beyond the Pines together, the stars "have been friends for a long time," a source explains to Us. "There's always been this strong chemistry between them...this was just a matter of time!"